Emerging Membrane Tech Glossary: ED, CDI & More

This glossary covers key terms related to electrochemical membrane processes, thermal and vapour-driven membrane processes, gas separation and specialty membrane applications. Each term is defined with technical accuracy for membrane researchers, engineers, and water treatment professionals. For the complete glossary covering all 80+ membrane technology terms, see our comprehensive membrane technology glossary.

Electrochemical Membrane Processes

Electrodialysis (ED)

An electrochemical membrane process that uses an electric field to drive ions through ion-exchange membranes (cation and anion exchange membranes arranged alternately) to separate salts from water. ED is used for brackish water desalination, acid and base recovery, and food processing. It is particularly effective for streams with moderate salinity levels.

Reverse Electrodialysis (RED)

An energy-generating process that converts salinity gradient energy into electricity by passing high-salinity and low-salinity streams through alternating cation and anion exchange membranes. Ions moving through the membranes generate an ionic current that is converted to electrical current at the electrodes. RED is a promising renewable energy technology for river–sea water interfaces.

Capacitive Deionisation (CDI)

An emerging desalination technology that removes ions from water by adsorbing them onto electrically charged porous carbon electrodes. When a voltage is applied, cations and anions are attracted to and stored on the negative and positive electrodes respectively. CDI is energy-efficient for low-salinity brackish water treatment. Tech Inc. provides CDI research cells and electrode testing equipment.

Membrane Capacitive Deionisation (MCDI)

An advanced variant of CDI that incorporates ion-exchange membranes in front of the carbon electrodes to improve charge efficiency and prevent co-ion expulsion during the charging phase. MCDI achieves higher salt removal rates and better energy efficiency compared to conventional CDI, making it promising for brackish water desalination.

Ion Exchange Membrane

A polymer membrane that selectively allows either cations (cation exchange membrane, CEM) or anions (anion exchange membrane, AEM) to pass through while blocking ions of the opposite charge. Ion exchange membranes are critical components in electrodialysis, reverse electrodialysis, fuel cells, and flow batteries. They are characterised by their ion exchange capacity, selectivity, and electrical resistance.

Bipolar Membrane

A composite membrane consisting of a cation exchange layer and an anion exchange layer laminated together. Under an applied electric field, water dissociates at the interface into hydrogen and hydroxide ions, enabling the production of acids and bases from salt solutions without chemical addition. Bipolar membranes are used in food processing, chemical manufacturing, and CO2 capture.

Thermal and Vapour-Driven Membrane Processes

Membrane Distillation (MD)

A thermally driven separation process where a hydrophobic microporous membrane separates a warm feed solution from a cooler permeate side. Only water vapour passes through the membrane pores, while liquid water and dissolved solutes are retained. MD can treat highly saline solutions beyond the limits of RO and can utilise low-grade waste heat or solar thermal energy.

Pervaporation

A membrane separation process used to separate liquid mixtures by partial vaporisation through a dense, selective membrane. The feed is in liquid contact with one side of the membrane, while a vacuum or sweep gas is applied on the permeate side to remove the vapour. Pervaporation is widely used for dehydrating organic solvents and removing volatile organic compounds from water. Tech Inc. supplies pervaporation test cells designed for accurate evaluation of membrane selectivity and flux.

Vapour Permeation

Similar to pervaporation, but the feed is supplied as a vapour rather than a liquid. The membrane selectively permeates certain vapour components based on their solubility and diffusivity in the membrane material. Vapour permeation is used in petrochemical processing for separating close-boiling-point mixtures and for gas dehydration applications.

Direct Contact Membrane Distillation (DCMD)

The simplest membrane distillation configuration where the hot feed solution and cold permeate are both in direct contact with the membrane surfaces. The temperature difference creates a vapour pressure gradient that drives water vapour through the hydrophobic membrane pores. DCMD is the most studied MD configuration due to its simplicity.

Air Gap Membrane Distillation (AGMD)

A membrane distillation configuration featuring an air gap between the membrane and a condensation surface on the permeate side. The air gap provides additional insulation to reduce heat conduction losses, improving thermal efficiency compared to DCMD. AGMD is well-suited for desalination and applications requiring high thermal efficiency.

Sweep Gas Membrane Distillation (SGMD)

A membrane distillation configuration where an inert gas sweeps across the permeate side of the membrane to carry away the water vapour to an external condenser. SGMD combines reduced heat loss with effective vapour removal, though it requires additional equipment for condensation and gas recirculation.

Gas Separation and Specialty Membrane Applications

Gas Separation Membrane

A dense or mixed-matrix membrane that selectively permeates certain gases based on differences in solubility and diffusivity within the membrane material. Gas separation membranes are used commercially for nitrogen generation from air, hydrogen recovery, CO2 capture, and natural gas sweetening. Membrane performance is characterised by permeability and selectivity for the target gas pair.

Organic Solvent Nanofiltration (OSN)

A membrane process for separating dissolved solutes from organic solvents, using membranes that are stable in non-aqueous environments. OSN is used in pharmaceutical purification, catalyst recovery, and solvent exchange. Membrane materials for OSN must resist swelling and degradation by the organic solvents being processed.

Membrane Contactor

A device that uses a microporous membrane to provide a controlled interface between two phases (typically gas-liquid or liquid-liquid) without direct mixing. The membrane acts as a barrier that maintains phase separation while allowing mass transfer. Applications include dissolved gas removal, carbon dioxide absorption, and blood oxygenation.

Mixed Matrix Membrane (MMM)

A composite membrane consisting of inorganic filler particles (such as zeolites, MOFs, or carbon nanotubes) dispersed within a polymer matrix to enhance separation performance. MMMs aim to combine the processability of polymers with the superior selectivity and permeability of inorganic materials, particularly for gas separation applications.

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Frequently Asked Questions

What is Electrodialysis (ED)?

An electrochemical membrane process that uses an electric field to drive ions through ion-exchange membranes (cation and anion exchange membranes arranged alternately) to separate salts from water. ED is used for brackish water desalination, acid and base recovery, and food processing. It is particularly effective for streams with moderate salinity levels.

What is Reverse Electrodialysis (RED)?

An energy-generating process that converts salinity gradient energy into electricity by passing high-salinity and low-salinity streams through alternating cation and anion exchange membranes. Ions moving through the membranes generate an ionic current that is converted to electrical current at the electrodes. RED is a promising renewable energy technology for river–sea water interfaces.

What is Capacitive Deionisation (CDI)?

An emerging desalination technology that removes ions from water by adsorbing them onto electrically charged porous carbon electrodes. When a voltage is applied, cations and anions are attracted to and stored on the negative and positive electrodes respectively. CDI is energy-efficient for low-salinity brackish water treatment. Tech Inc. provides CDI research cells and electrode testing equipment.

What is Membrane Capacitive Deionisation (MCDI)?

An advanced variant of CDI that incorporates ion-exchange membranes in front of the carbon electrodes to improve charge efficiency and prevent co-ion expulsion during the charging phase. MCDI achieves higher salt removal rates and better energy efficiency compared to conventional CDI, making it promising for brackish water desalination.

What is Ion Exchange Membrane?

A polymer membrane that selectively allows either cations (cation exchange membrane, CEM) or anions (anion exchange membrane, AEM) to pass through while blocking ions of the opposite charge. Ion exchange membranes are critical components in electrodialysis, reverse electrodialysis, fuel cells, and flow batteries. They are characterised by their ion exchange capacity, selectivity, and electrical resistance.

What is Bipolar Membrane?

A composite membrane consisting of a cation exchange layer and an anion exchange layer laminated together. Under an applied electric field, water dissociates at the interface into hydrogen and hydroxide ions, enabling the production of acids and bases from salt solutions without chemical addition. Bipolar membranes are used in food processing, chemical manufacturing, and CO2 capture.

What is Membrane Distillation (MD)?

A thermally driven separation process where a hydrophobic microporous membrane separates a warm feed solution from a cooler permeate side. Only water vapour passes through the membrane pores, while liquid water and dissolved solutes are retained. MD can treat highly saline solutions beyond the limits of RO and can utilise low-grade waste heat or solar thermal energy.

What is Pervaporation?

A membrane separation process used to separate liquid mixtures by partial vaporisation through a dense, selective membrane. The feed is in liquid contact with one side of the membrane, while a vacuum or sweep gas is applied on the permeate side to remove the vapour. Pervaporation is widely used for dehydrating organic solvents and removing volatile organic compounds from water. Tech Inc. supplies pervaporation test cells designed for accurate evaluation of membrane selectivity and flux.

What is Vapour Permeation?

Similar to pervaporation, but the feed is supplied as a vapour rather than a liquid. The membrane selectively permeates certain vapour components based on their solubility and diffusivity in the membrane material. Vapour permeation is used in petrochemical processing for separating close-boiling-point mixtures and for gas dehydration applications.

What is Direct Contact Membrane Distillation (DCMD)?

The simplest membrane distillation configuration where the hot feed solution and cold permeate are both in direct contact with the membrane surfaces. The temperature difference creates a vapour pressure gradient that drives water vapour through the hydrophobic membrane pores. DCMD is the most studied MD configuration due to its simplicity.